Most typically, the preparation of carbonated ice has in its simplest form entailed contacting an aqueous liquid with gaseous carbon dioxide under highly elevated pressures for a period of time sufficient to form a desired level of carbon dioxide hydrate, and then cooling to freeze the resulting product. Such a gas-liquid contact process is disclosed by Barnes et al in U.S. Pat. Nos. 2,975,603; 3,086,370; and 3,217,503.
According to the specific example disclosed in the first-mentioned Barnes et al patent, water was contacted with gaseous carbon dioxide at a pressure of 400 psig at a temperature just above 0.degree. C. After 75 minutes, the vessel was removed from the bath, cooled to freeze the contents, depressurized and opened. Because the entire contents of the reactor must be frozen to achieve a stable product, and the heat transfer characteristics of the aqueous suspension of carbon dioxide hydrate within the reactor are inferior to pure water, freezing is inefficient. Additionally, the freezing must be conducted under a suitably elevated pressure; and in the case of these Barnes et al patents, freezing is conducted within the reactor vessel itself. This freezing step, therefore, severely limits productivity for a given reactor volume. The disclosures of the other two Barnes et al patents are similar in this regard. Moreover, pure hydrate could not be prepared.
Adler et al stated in U.S. Pat. No. 3,220,204, that, while the prior art procedures of Barnes et al produce carbonated ice which retains significantly high levels of carbonation during refrigerated storage, the carbonated ice had a tendency to explode or pop (i.e., break apart and disintegrate with a loud noise) at an unpredictable point of time during dissolution. They indicated when the Barnes et al carbonated ice products were added to water or milk, they frequently exploded in the glass. Their solution to the problem entailed preparing a suspension of hydrate in aqueous liquid in one vessel by maintaining a high liquid surface to gas contact during reaction, and then transferring the suspension to a separate freezing zone. It is disclosed that, as a practical matter, in order to operate under controllable conditions, hydrate will be produced at pressures above 200 psig and a temperature above 0.degree. C. in order to maximize hydrate formation while minimizing collateral formation of water ice. Like the process of Barnes et al, the unreacted aqueous liquid had to be frozen while under pressure. Moreover, transfer to the freezing zone was difficult where high levels of carbon dioxide were entrapped within the product. And, again, pure hydrate could not be prepared even though they suggested removing all possible water by pressing.
In U.S. Pat. No. 3,333,969, Mitchell et al disclosed that the problem of uneven release of carbon dioxide persisted throughout all prior art gasified ice products. They indicated that an important problem present in the handling and use of carbonated ice, particularly in the range of from 10 to 118 volumes of CO.sub.2 per gram of ice, was an uneven release of carbon dioxide from the carbonated ice. To eliminate the problems of popping and splashing, Mitchell et al proposed subdividing carbonated ice into discrete particles while maintaining the temperature of the ice below 0.degree. C., and then contacting the discrete particles to form them into an adhered mass or briquette. Briquetting produced a gasified ice product having a commercially satisfactory mechanical strength in the frozen state and also liberated entrained gas bubbles which are believed to cause the undesirable, spontaneous popping, and exploding phenomena. This process improved the uniformity of the end-product, but did not address the problem of the inefficiency of the freezing step, and the briquetting step actually reduced the level of gas in the final product and increased the rate of loss of CO.sub.2.
In U.S. Pat. No. 3,255,600, Mitchell et al disclosed that liquid carbon dioxide could be employed in place of gaseous carbon dioxide for preparing a carbonated ice product. According to the disclosed process, liquid carbon dioxide and either liquid water or water ice are mixed under controlled conditions to form a carbonated ice product which is then cooled to below the freezing point, preferably by simply venting carbon dioxide gas to the atmosphere and taking advantage of the cooling effect of the expanding gas. According to claim 3, a carbonated ice product characterized by a high carbon dioxide content and a long storage life is prepared by initially mixing liquid carbon dioxide with ground water ice in a closed reaction vessel. The head space in the vessel is maintained at a pressure above 500 psig while the mixture is agitated. The temperature of the mixture rises from 0.degree. C. to a maximum of approximately 11.degree. C. and the pressure in the reaction vessel increases to a maximum of approximately 655 psig during the course of the reaction period. The reaction is continued until the pressure and temperature values start to decrease. The disclosure indicates that venting produces a carbonated ice product in the form of a highly carbonated snow in 30 seconds. However, because the reaction temperature is above the freezing point of the unreacted water, the entire reaction mixture must be frozen to stabilize the resulting product. And, the vaporization of liquid carbon dioxide to provide cooling is very energy intensive and inefficient.
The literature has also suggested the possibility that carbon dioxide hydrate could be formed by a gas-solid process at extremely low temperatures e.g., about -70.degree. C. to -40.degree. C. However, these references indicated a strong pressure dependence on the stability of the products; and, the decomposition rates, if extrapolated to those encountered in normal home or commercial refrigeration equipment, would be enormously high.
Miller and Smythe in Science, Vol. 170, Oct. 1970, Pages 531-533, discussed the formation of carbon dioxide hydrate by a gas-solid process at -73.degree. C. to -43.degree. C. and studied the kinetics of decomposition between -121.degree. C. and -101.degree. C. To prepare the hydrate, finely-divided ice was obtained by grinding ice under liquid nitrogen with a mortar and pestle and by condensing water from air at -195.degree. C. The ice was degassed at -43.degree. C., and hydrate was prepared in a vacuum line at temperatures between -73.degree. C. and -43.degree. C. The temperature was then brought to the desired value for studying decomposition, and the equilibrium was approached from both the high- and low-pressure sides of the dissociation pressure.
Adamson and Jones, in Journal of Colloid and Interface Science, Volume 37, No. 4, December 1971, at Pages 831-835, also studied the preparation of carbon dioxide hydrate at low temperatures. For samples of ice prepared by quenching a hot steam jet in liquid nitrogen, a strongly-pressure-dependent absorption region was identified within the range of from about -83.degree. C. to -73.degree. C. They indicated that their data was consistent with that of Miller and Smythe.
It is apparent from the foregoing discussion of the prior art that studies were made in temperature ranges much lower than those disclosed in this invention and the anomalous increase in the rate of reaction at temperatures below the freezing point of water was never observed. Thus, the prior art has not enabled the preparation of high purity, high gas content hydrates other than by the use of processes which are inefficient in terms of energy consumption, reaction rates or projected product stability under ordinary storage temperatures.
Experience shows that the step of freezing the reaction product of water or water ice reacted at temperatures above 0.degree. C. requires the use of equipment which is less than optimum in design than that which is presently commercially available for freezing water ice at ordinary temperatures.
Where the prior art has employed water ice as a starting material, it was necessary, in the case of U.S. Pat. No. 3,255,600, to carry out the reaction at a temperature above the freezing point of water ice, and to still require a significant amount of cooling to refreeze the reaction mixture.
Where water ice has been reacted with gaseous carbon dioxide in the past, the reactions have progressed slowly and have been conducted at excessively low temperatures--requiring further expenditure of energy. This prior art indicates that products prepared by contacting water ice with gaseous carbon dioxide would be highly unstable when elevated to temperatures more typically encountered in home and commercial refrigeration devices.
Thus, there remains a current need for a process capable of producing high-gas-content or essentially-pure gas hydrate with increased efficiency.